In recent years, a variety of optical recording media have been developed and optical pickup devices that carry out recording and reproducing using two alternative types of optical recording media have been known. For example, devices that carry out recording or reproducing with either a DVD (Digital Versatile Disk) or a CD (Compact Disk including CD-ROM, CD-R, CD-RW) have been used. For these two optical recording media, the DVD uses visible light having a wavelength of approximately 657 nm for improved recording densities while the CD is required to use near-infrared light having a wavelength of approximately 790 nm because there are some recording media that have no sensitivity to visible light. A single optical pickup device, known as a two-wavelength-type pickup device, uses incident light of these two different wavelengths. The two optical recording media described above require different numerical apertures (NA) due to their different features. For example, the DVD is standardized to use a numerical aperture of about 0.65 and the CD is standardized to use a numerical aperture in the range of 0.45-0.52. Additionally, the thicknesses of the two types of recording disks, including the thicknesses of the protective layers or substrates made of polycarbonate (PC), are different. For example, the DVD may have a substrate thickness of 0.6 mm and the CD may have a substrate thickness of 1.2 mm.
As described above, because the substrate thickness of the optical recording medium is standardized and differs according to the type of optical recording medium, the amount of spherical aberration introduced by the substrate is different based on the different standardized thicknesses of the substrates of the different recording media. Consequently, for optimum focus of each of the light beams on the corresponding optical recording medium, it is necessary to optimize the amount of spherical aberration in each light beam at each wavelength for recording and reproducing. This makes it necessary to design the objective lens with different focusing effects according to the light beam and recording medium being used.
Additionally, in response to rapid, almost daily, increases of data capacity, the demand for an increase in the recording capacity of recording media has been strong. It is known that the recording capacity of an optical recording medium can be increased by using light of a shorter wavelength and by increasing the numerical aperture (NA) of an objective lens. Concerning a shorter wavelength, the development of a semiconductor laser with a shorter wavelength using a GaN substrate (for example, a semiconductor laser that emits a laser beam of 408 nm wavelength) has advanced to the point where this wavelength is now available for use.
With the development of short wavelength semiconductor lasers, research and development of AODs (Advanced Optical Disks), also known as HD-DVDs, that provide approximately 20 (GB of data storage on a single layer of a single side of an optical disk by using short wavelength light is also progressing. As the AOD standard, the numerical aperture and disk thickness are selected to be close to, but slightly different from, those of DVDs, with the numerical aperture (NA) and disk substrate thickness for an AOD being set at 0.65 and 0.6 mm, respectively.
Furthermore, research and development of Blu-ray disk (BD) systems that use a shorter wavelength of disk illuminating light, similar to AOD systems, has progressed, and the standardized values of numerical aperture and disk thickness for these systems are completely different from the corresponding DVD and CD values, with a numerical aperture (NA) of 0.85 and a disk substrate thickness of 0.1 mm being standard. Unless otherwise indicated, hereinafter, AODs and Blu-ray disks collectively will be referred to as “AODs.”
The development of an optical pickup device that can be used for three different types of optical recording media, such as AODs, DVDs and CDs as described above, has been demanded and objective optical systems for mounting in such devices have already been proposed. For example, an objective optical system that includes a diffractive optical element with a refractive surface and a diffractive surface and a biconvex lens is described on page 1250 of Extended Abstracts, 50th Japan Society of Applied Physics and Related Societies (March, 2003). The objective optical system described in this publication is designed so that: second-order diffracted light from the diffractive optical element is used for a BD optical recording medium; first-order diffracted light from the diffractive optical element is used for a DVD optical recording medium; and also first-order diffracted light from the diffractive optical element is used for a CD optical recording medium. The rear surface of the diffractive optical element (the side opposite the light source) is concave in order to aid in correcting spherical aberration that is created by the difference in the thickness of the protective layer, that is, the substrate of each optical recording medium. The spherical aberration created varies with the thickness of the protective layer. Chromatic aberration is also improved relative to a single component lens by the diffractive optical element having the diffractive surface as its front surface, that is, the surface on the light source side, and the concave surface as its rear surface.
In the technology described in the above-mentioned publication, in order to reduce the generation of coma associated with a shift of the objective optical system relative to an incident light beam, when recording or reproducing information to or from the BD, the design is such that the light incident on the diffractive optical element is converging light. Further, when recording or reproducing information to or from the DVD or the CD, the design is such that the light incident on the diffractive optical element is collimated light and diverging light, respectively.
However, there presently is strong demand for a compact device that provides greater freedom in positioning the objective optical system within the recording and reproducing device. In order to achieve this, it is necessary to create a design such that collimated light, rather than diverging or converging light, be incident on the objective optical system for all three of the light beams that are being used. Additionally, if diverging or converging light is incident on the diffractive optical element, there are problems of the diffraction efficiency being reduced due to the angle of incidence of the light rays on the diffractive grooves of the diffractive optical element being tilted from the desired angle of incidence, and there are problems of the stability of the tracking being decreased.